Wurrumph? Canāt a man get some sleep around here? Uh, 'morning, Jon.
I donāt find that much difference in the sound of lined and unlined heads, unless differential movement has caused a little bit of the slide to protrude into the embouchure hole. Then they sound really raspy. I suspect this might be the reason why people like Quantz condemned the lined head at the time, and why people used to a raspy head these days are often pleasantly surprised when they try an unlined head.
The real reason behind lined heads was the tuning slide. When Richard Potter introduced it in the late 18th century, he chose to do it in the form we are all now familiar with - lined barrel and lined head, with the liners forming the tuning slide at their junction. Probably seemed a good idea at the time, and of course central heating was not in the picture for nearly 200 years.
Itās interesting to compare the 18th century flutes (no metal liners or tuning slides) and the 19th century flutes (metal liners in head and barrel forming a tuning slide at their junction) in the museums. The 19th century flutes are almost allways cracked in both head and barrel, while the earlier unlined instruments are almost always fine.
For whatever reason, the French didnāt take up the fully lined head idea. (I havenāt tried it, but it might make a short D foot French flute unacceptably head-heavy). So their old flutes tend to have cracked barrels and only partially cracked heads.
For the new Boehm flutes, the English had very short slides hidden in the tenons and sockets. Old ones of these are usually only cracked at the tenons and sockets. Notice a trend? Wherever it is lined, it cracks.
And itās not only tuning slides, any imprisoned metal can be enough to promote cracking. Iāve just finished up fixing an early 19th century flute that had slot liners in all the key slots. All but one of these had promoted a crack in the body of the flute underneath it. The one that didnāt was the short F block - that one was safe because there is no appreciable shrinkage along the grain in timber.
It has been argued that blackwood is strong enough to withstand the forces of shrinkage, but weāre now starting to see quite a few blackwood flutes cracking.
And it has been argued that adequate seasoning is enough, but the figures donāt support that. If you take a piece of wood that was adequately seasoned to the 65% RH in the British Isles to a typical inland destination in America or Australia (average RH 35%) the wood around the slide will try to shrink about 0.3mm in diameter, about 1mm in circumference, which is a lot to ask.
Youāre probably all familiar with my āNew Improved Tuning Slideā design, which employs partial lining AND a cork buffer arrangement to get around the problem. You can see the full details including a quaint little sketch at:
http://www.mcgee-flutes.com/fluteslide.html
Some other makers have strategies that they hope work, others stick to the traditional method and expect you to keep the humidity up to the flute. Important obviously to know what approach your maker has taken and what if any precautions they expect you to take. Important all the more if youāre contemplating buying a maker from a place with a radically different climate to yours.
Hmmm, Iām awake now. Better go and make a flute, I guess ā¦
Terry
Then closer scrutiny revealed 2 additional cracks in the flute head. 
To find this on a flute described as āPRISTINEā was quite upsetting! 
I suspect the cracks formed during transit from London U.K. to the central U.S. I would recommend extreme precautions be taken to prevent any such occurrence. The flute was well protected in its packaging. But was not oiled nor the case humidified and sealed prior to shipping.
